1. Field of the Invention
The present invention relates to honeycomb structure bodies having a structure in which the honeycomb structure body has partition walls to form a plurality of cells, and a radial cross section of the honeycomb structure body, perpendicular to an axial direction along which the cells are formed, is divided into a plurality of cell density sections having different cell densities, and the cells belong to respective cell density sections.
2. Description of the Related Art
There have been known and widely used honeycomb structure bodies as a catalyst supporter, in which catalyst is supported. A honeycomb structure body is mounted to an exhaust gas pipe connected to an internal combustion engine such as a diesel engine and purifies exhaust gas emitted from the internal combustion engine. For example, such a honeycomb structure body has an outer peripheral wall of a cylindrical shape, and partition walls arranged in a lattice shape in the inside of the outer peripheral wall. In particular, the partition walls are formed and arranged in a lattice shape to form a plurality of cells along an axial direction of the honeycomb structure body. Each of the cells is surrounded by the partition walls. The cells are formed along an axial direction of the honeycomb structure body.
The honeycomb structure body having such a structure is mounted to the exhaust gas pipe connected to an internal combustion engine. Through the exhaust gas pipe, exhaust gas emitted from the internal combustion engine is discharged outside. Because exhaust gas has a high temperature, the catalyst supported in the honeycomb structure body is activated by heat energy of the exhaust gas and the activated honeycomb structure body at a high temperature purifies the exhaust gas when the exhaust gas is passing through the inside of the cells in the honeycomb structure body.
The recent vehicle emission control is becoming stricter year by year in view of environmental protection, and there is also strong demand to decrease carbon dioxide contained in exhaust gas emitted from internal combustion engines such as diesel engines and more improve fuel efficiency of motor vehicles. It is very important to eliminate particulate matter such as black smoke contained in exhaust gas emitted from diesel engines in view of recent vehicle emissions control which intends to reduce motor vehicle emissions, etc. In order to solve and satisfy the above recent requirement, many motor vehicles are equipped with a diesel particulate filter. The diesel particulate filter equipped with a honeycomb structure body is mounted on an exhaust gas pipe through which exhaust gas emitted from a diesel engine is discharged to the outside of the motor vehicle. The diesel particulate filter purifies such exhaust gas. For this reason, this increases an amount of the noble metal used as catalyst in the honeycomb structure body. In order to avoid price risk of the noble metal and resource procurement risk, it is required to reduce a total amount of the noble metal used in the honeycomb structure body. Accordingly, there is a strong demand for a honeycomb structure body to have an excellent purification capability of exhaust gas.
For example, a patent document, Japanese patent laid open publication No. 2013-173133 has disclosed a honeycomb structure body having a conventional structure in which a plurality of cell density sections having different cell densities is formed in a radial direction of the honeycomb structure body. This radial direction is perpendicular to an axial direction of the honeycomb structure body. The cell density sections are formed from a central section to the outer peripheral section in a radial cross section. Because the outer peripheral section has a low flowing speed of exhaust gas as compared with a flowing speed of the exhaust gas in the central section, a cell density is gradually reduced from the central section to the outer peripheral section in the honeycomb structure body disclosed in the patent document, Japanese patent laid open publication No. 2013-173133. This structure makes it possible to have a uniform flowing speed of exhaust gas in the honeycomb structure body, and promote an effective use of catalyst supported in the honeycomb structure body. The honeycomb structure body having the structure previously described improves the exhaust gas purification capability.
Still further, a boundary section is further formed between the cell density sections having different cell densities in the honeycomb structure body disclosed in the patent document, Japanese patent laid open publication No. 2013-173133. Because the boundary section has boundary cells having a specific structure, it is possible to reduce a pressure loss of the honeycomb structure body and increase the exhaust gas purification capability.
However, the boundary section formed between the adjacent cell density sections having different cell densities in the honeycomb structure body previously described has an insufficient strength around the boundary section, and there is a possible drawback of the honeycomb structure body easily breaking due to external stress. Furthermore, the honeycomb structure body is generally produced by firing a honeycomb mold body having a honeycomb structure. The boundary section formed between the cell density sections in the honeycomb structure body non-uniformly shrinks during a drying step and a firing step in the manufacturing process of producing honeycomb structure bodies. For this reason, it is difficult to produce a honeycomb structure body having a correct roundness. This reduces a productivity of honeycomb structure bodies having a correct roundness.
That is, it is possible to increase the strength of the honeycomb structure body when a boundary wall section is formed between two cell density sections having different cell densities, and reduce influence of outside stress applied to the honeycomb structure body. However, because small sized cells having a small cross section are formed near the boundary wall section, catalyst clogging (in which a cell is clogged with catalyst) occurs in the small sized cells when catalyst is supported in the honeycomb structure body during the manufacturing process of producing honeycomb structure bodies. As a result, such small sized cells having a small cross section formed adjacent to the boundary wall section are clogged with catalyst. This increases a pressure loss of the honeycomb structure body. In addition, the formation of catalyst-clogged cells causes a reduction in exhaust gas flow in some sections which support catalyst and are clogged by catalyst. This is a waste of catalyst because catalyst is used in the unnecessary sections in the honeycomb structure body.